1. Field of the Invention
The invention relates to electronic circuits having light emitting devices, and more particularly, to a light emitting device driver for driving a light emitting device and an integrated circuit integrating a light emitting device and a light emitting device driver.
2. Description of the Prior Art
Light emitting diodes (LEDs) are semiconductor devices that convert electrical energy directly into light. The emitted light is due to the nature of the bonding that occurs in the semiconductor solid. As is well known, the type of bonding in a solid is directly related to the conductivity of the solid. Metals, nonmetals, and semimetals have different bonding properties that lead to the differences in conductivity that can be observed between these categories of elements. LEDs rely on special conductivity properties in order to emit light, and operate by a completely different mechanism from other sources of light, such as light bulbs and the sun.
Furthermore, as LEDs generally produce very little heat, LEDs are much more efficient for producing light than other light sources. Because batteries provide only a limited amount of energy, reduced energy consumption is very beneficial to battery operated portable electronic devices. As such, LEDs are often used as indicator lights or other light sources for portable electronic devices such as mobile phones, notebook computers, personal digital assistants (PDAs), etc.
FIG. 1 shows a schematic diagram of a first typical LED driver circuit driving a plurality of LEDs 110 connected in series to emit light. The first typical LED driver circuit is an inductive boost circuit including an input capacitor 102, a switching regulator 104, an inductor 106, a diode 108, an output capacitor 112, and a load resistor 114. As will be well understood by a person of ordinary skill in the art, the switching regulator 104 charges the inductor 106 at a particular switching frequency to boost an input voltage VIN and thereby generate an output voltage VOUT having a higher voltage. This higher output voltage VOUT is capable of driving the plurality of LEDs 110 connected in series to emit light. A control signal CTRL is used to either enable or disable the switching regulator 104 and thereby turn on or off the plurality of LEDs 110.
FIG. 2 shows a schematic diagram of a second typical LED driver circuit 200 for driving a plurality of LEDs 210 connected in series to emit light. The second typical LED driver circuit 200 is a charge pump circuit including an first capacitor 202, a second capacitor 203, a third capacitor 204, a load resistor 206, and a plurality of switches S1–S4. As will be well understood by a person of ordinary skill in the art, the plurality of switches S1–S4 are toggled at a particular switching frequency to boost an input voltage VIN and thereby generate an output voltage VOUT having a higher voltage. Similar to the first typical LED drvier circuit 100 shown in FIG. 1, the higher output voltage VOUT in FIG. 2 is capable of driving the plurality of LEDs 210 connected in series to emit light.
However, the first and second typical LED driver circuits shown in FIG. 1 and FIG. 2 both suffer from similar problems. These problems include switching noise and high component requirements. More specifically, concerning the switching noise, as the switching regulator 104 or the plurality of switches S1–S4 switch on and off, sudden changes in current drawn from the supply voltages (VIN, GND) causes noise to appear on the supply voltages (VIN, GND). This switching noise adversely affects other circuit components and must be reduced, particularly in very compact and therefore noise sensitive portable electronic devices such as mobile phones. Additionally, both the inductive boost circuit structure of FIG. 1 and the charge pump circuit structure of FIG. 2 require significant numbers of external components such as capacitors, diodes, and inductors. These external components not only increase the cost and the required implementation size of the circuit, but also increase the overall design complexity and development time of products requiring LEDs.
FIG. 3 shows a typical circuit structure for directly driving a plurality of LEDs 300 to emit light without first boosting an input voltage VIN. In this situation, the LEDs 300 are connected in parallel to eliminate the need to boost the input voltage VIN. Although the circuit structure of FIG. 3 partially solves the above-mentioned problems, the circuit structure of FIG. 3 is unable to drive the LEDs 300 to emit light when a voltage drop across the resistor 304 plus the forward voltage drop across the parallel combination of LEDs 300 is greater than the input voltage VIN. For example, to limit the current flowing through each diode to an appropriate amount, there is typically at least a 0.1 V voltage drop across the resistor 304. Therefore, if low forward voltage LEDs 300 having a forward voltage drop of 3.3V are used, the circuit structure of FIG. 3 will only operate while the input voltage VIN is above 3.4V.
FIG. 4 shows a typical battery discharge graph of battery voltage vs. operating time of a lithium-ion (Li-ion) battery. Li-ion batteries are often used in such portable electronic devices as mobile phones and notebook computers, and as previously mentioned, LEDs are often incorporated into these devices. As shown in FIG. 4, when fully charged, the Li-ion battery has a voltage of approximately 4.1 V. Over time, when about 30% of the energy in the battery is used, the voltage drops to approximately 3.7V; and then when about 80% of the energy in the battery is used, the voltage again begins to significantly drop toward 3.0V. Therefore, assuming a voltage drop across the resistor 304 of 0.1 V, as soon as the voltage of the Li-ion battery drops below 3.4V, indicated at point A in FIG. 4, the LEDs 300 of the circuit of FIG. 3 will no longer emit light. Therefore, the LEDs 300 will not function for the last few percent of Li-ion battery operating time. It would be beneficial to be able to drive LEDs to emit light at lower input voltages (VIN) while minimizing external components and avoiding switching noise.